With the advent of personal computers, the need for interconnectivity between computational platforms has grown exponentially. In the business environment, connectivity is necessary for transmittal of email and numerous other types of data including audio and video. In the residential area, many homes are now connected via the Internet. All of this interconnectivity requires increasingly higher throughput across both wired and wireless networks.
The transmission of high data rates, such as for video signals, requires expensive connections when performed over wired networks. For temporary applications, the installation cost becomes prohibitive. The most economical solution is to transmit data via satellites. This method eliminates the high cost of connecting any location with coaxial wire or fiber optics to the nearest telephone switching office.
Existing satellites are equipped with several transponders that relay information. Each transponder typically has a pass-band of 36 megahertz. Using present modulation and coding systems, these transponders can handle a data rate of nearly 70 megabits/second. Satellites operating in the Ka band are expected to be available to the marketplace in the next 2–3 years. These Ka band transponders will support pass-bands of 100 to 500 megahertz.
On the ground, existing networking equipment runs at established fixed speeds. These speeds include (in megabits/second): 1.5, 45, 155, and 622. To connect a satellite communications network with ground-based network, the satellite modem must operate at lower speeds that are identical to ground-based speeds. This severely limits the efficiency of the satellite's available bandwidth.
The satellite connection is handled through a satellite modem. Current satellite modems use a serial input/output such as RS-449. A translation device is required to connect the standards based ground network to the satellite modem. If the satellite is operating at a similar speed, it can be effectively connected using a commercial router. To operate efficiently at any other speed, the connection must be made through a different interface device.
The prior art includes three types of interface devices. All of these devices convert the satellite modem serial data to/from the standards based protocols of Asynchronous Transmission Mode (ATM) or Ethernet. These devices, two of which are commercial and one of which is a governmental (NASA) design, include:
1) The COMSAT Link Accelerator (CLA-2000) converts RS-449 serial interface at up to 8 megabits/second to an ATM interface at a fixed 45 megabits/second. The CLA-2000 was recently upgraded to support 10 megabits/second Ethernet in addition to ATM.
2) The Metrodata LA-1000 converts, an Asynchronous Serial Interface (ASI) to ATM. The LA-1000 interfaces between satellite modems with DVB-ASI interfaces and ATM networks at speeds up to about 100 megabits/second. The LA-1000 does not support satellite modems with Emitter Coupled Logic (ECL) interfaces.
3) The NASA Goddard Space Flight Center's Ground Router Interface Device (GRID) includes the features of the CLA-2000 and the capability to support multiple RS-449 interfaces. This device is also limited to 8 megabits/second on the terrestrial serial interface. Details of GRID are proprietary and not publicly available. Current systems in the ECL domain are thus limited to 8 megabits/second.
Referring to FIG. 1, there is shown a simplified block diagram of a current satellite communication system 10 for communicating with the International Space Station (ISS) 12. The ISS 12 communicates with a ground station 16, which in the present case is NASA's White Sands Complex (WSC), via a Tracking & Data Relay Satellite (TDRS) 14. The uplink from the ground station 16 to TDRS 14 is at a data rate of 3 megabits per second (Mb/s), while the downlink to the ground station is at the rate of 50 Mb/s. The Ku-SA channel forward link from TDRS 14 to ISS 12 is 50 MHz wide, while the Ku-SA channel return link from ISS to TDRS is 225 MHz wide. The TDRS ground system 16 does not afford a Forward Error Correction (FEC) capability, which limits the capability of this link. The downlink signal received from TDRS 14 undergoes frequency translation in the ground station 16 and is provided to a TDRS modem 18 which demodulates the 50 Mb/s signal down to a baseband signal which includes an ECL clock signal and the received data. The ECL clock signal and the data are provided to a high rate switch 20, which is adapted for interfacing with the combination of a domestic satellite (DOMSAT) 26 and DOMSAT modem 24 within the ground station 16. An “Air Gap” is illustrated in the ground station 16 between the high rate switch 20 and a SONET mux 22 to illustrate that there is currently no available means for linking a 50 Mb/s signal to a conventional ground communication network such as SONET. The ECL clock signal and data are provided from the high rate switch 20 to the domestic satellite 26 via the DOMSAT modem 24. Domestic satellite 26 is a commercial spacecraft operating in the Ku band, which receives signals and retransmits the signals at a data rate of 50 Mb/s. The domestic satellite 26 retransmits the data in a downlink to an earth facility such as NASA's Marshall Space Flight Center (MSFC) 28 and NASA's Johnson Space Center (JSC) 30 in the form of synchronous serial data. This satellite communications network is government proprietary and thus not available to the general public, is expensive to operate, and suffers from an extensive signal delay in the link between ground station 16 and the NASA space centers 28 and 30 via the domestic satellite 26, e.g., an average delay of 270 ms.
Another disadvantage of this and other prior approaches is that satellites are forced to operate at data rates dependent on the terrestrial network components at either end of the link. Often, the ideal data rates of the wireless satellite network segment lie in between the supported terrestrial data rates. To guarantee error-free connections via satellite, sufficient signal power must be provided to account for atmospheric disturbances such as clouds and rain. Reducing the data rates effectively eliminates the need to increase RF power levels to compensate. However, prior approaches do not allow for agile data rates.
Another disadvantage of prior approaches involves Forward Error Correction (FEC), which is a well-known tool for increasing communications link reliability. The high rate satellite modems 18 for Tracking & Data Relay Satellites (TDRS) do not employ Reed-Solomon FEC. To maximize the capability of these high rate connections, Reed-Solomon FED must be employed on the ISS 12 and at the high rate modem 18.
The present invention addresses the aforementioned limitations of the prior art by providing continuously adjustable input/output serial data rates via ECL-based satellite modems and converting the data rate to standards-based terrestrial network data rates with Reed-Solomon FEC capability at speeds up to 622 megabits/second.